1. Field of the Invention
This invention relates to a liquid phase growth process for producing semiconductor crystals and optical crystals of various types used in semiconductor devices and electro-optical devices, a substrate member production method making use of the process, and a liquid phase growth system suited for carrying out the process.
2. Related Background Art
With an increasing consciousness about environment, e.g., about air pollutions, solar cells have come to be widely used also for public use. Also, in such solar cells for public use, single-crystal or polycrystalline silicon is chiefly used as a semiconductor material. At present, these crystals are cut out of a large ingot in the shape of wafers each having a stated thickness of about 300 μm.
Such a method, however, gives forth cut waste of about 200 μm when each wafer is cut out, and hence utilizes materials in a poor efficiency. Moreover, it requires industrial-waste treatment for the part corresponding to the cut waste. Hereafter, in order to achieve higher production and achieve a low price further, it is desired to grow a crystal in the optically necessary smallest thickness and use such a crystal.
As a method for growing such thin crystal silicon, studies have hitherto been made on a gaseous-phase growth process in which a gas containing silicon is decomposed by the action of heat or plasma. In the mass production of solar cells, a system is demanded which can grow silicon at a rate of 1 μm/minute or higher on tens to hundreds of substrates of 4 to 5 inches square in one batch. However, any gaseous-phase growth system adaptable to such specification has not been put on the market.
As growth processes for crystals, besides the foregoing, a process called a liquid phase growth process is known from old times, and is actually utilized in the manufacture of compound semiconductor crystals for LEDs and optical crystals for electro-optical devices. Nowadays, as disclosed in Japanese Patent Application Laid-Open No. 10-189924, an example is reported in which a silicon crystal film grown on a crystal silicon substrate or a ceramic substrate is utilized for the manufacture of solar cells.
The liquid phase growth process is a process in which a metal such as tin, indium or gallium or an oxide such as lithium acid and niobic acid is heated to melt it, and a material for constituting the crystal, such as arsenic or silicon, is optionally further dissolved therein to form a melt, a substrate is immersed therein, and the melt is super-saturated by a means such as cooling to cause a crystal to deposit on the substrate.
This liquid phase growth process enables growth of crystals in good quality and moreover, compared with the gaseous-phase growth process, may less give force the material that is wasted without contributing to the growth of crystal. Accordingly, this process is suited for the application to devices required strongly to be inexpensive, such as solar cells, and to electro-optical devices in which expensive materials such as gallium and niobium are used.
The liquid phase growth process, however, has hitherto been limited in its use, and apparatus for growing compound semiconductors on substrates of 3 inches or smaller have only been put on the market. In particular, the process has been applied to the growth of silicon only a little.
Taking account of problems in conventional liquid phase growth processes and liquid phase growth systems, the present inventors have made studies on methods necessary for achieving the throughput that is demanded in mass production of solar cells, and on apparatus or systems suited for carrying out the processes.
More specifically, a conventional liquid phase growth system which can grow crystals on a plurality of substrates is constructed, e.g., as shown in FIG. 2. It has a substrate-supporting means (consisting basically of a supporting rack 202 and an up-and-down rod 209), and five substrates 201 are horizontally supported with its supporting rack 202 keeping stated intervals, and are immersed in a melt 204 held in a cylindrical crucible 203 provided in a growth heater 205. Here, the temperature of the melt 204 is appropriately controllable by an electric heater 206. The growth heater 205 is also fitted with a gate valve 207 so as to be opened or closed as occasion calls.
To grow crystals on the substrates 201 by using this growth system, first, dissolving substrates 201′ (denoted by reference numeral 201′ in order to distinguish it from the growth target substrates) comprised of a crystal material such as silicon are supported with the supporting rack 202 of the substrate-supporting means. These are then immersed in a solution in which a low-melting point metal such as indium or gallium or an oxide such as lithium acid and niobic acid has been dissolved and which has been heated to a stated temperature by the electric heater 206, and the crystal material is dissolved until it comes to stand saturated at that temperature, to prepare the melt 204.
Thereafter, the dissolving substrates 201′ are drawn up from the melt 204, and are changed for the growth target substrates 201 to be held in the supporting rack 202 (hence, in the drawing, the growth target substrates 201 and the dissolving substrates 201′ are not distinguished from each other). Thereafter, the melt 204 is gradually cooled. At the time it has reached a preset temperature, the supporting rack 202 now holding the growth target substrates 201 is descended to immerse the substrates 201 in the melt, whereupon over-saturated material having become not dissolved completely in the melt begins to deposit on the surface of the substrate 201. Thus, the crystal such as silicon grows on each substrate.
Incidentally, when the substrate 201 used here is polycrystalline or is glass or ceramic, the crystal is grown to be polycrystalline. When the substrate is single-crystal, it can be grown to be single-crystal.
Then, at the time the crystal has grown in a desired thickness, the supporting rack 202 holding the substrates 201 are drawn up. In this system, the substrates 201 are attached to or detached from the supporting rack 202 of the substrate-supporting means in the state the gate valve 207 is kept closed. The gate valve 207 is opened after the atmosphere has been displaced with an inert gas in a load lock chamber 208, and then the supporting rack 202 holding the substrates 201 is descended to the interior of the growth heater 205. Thus, the melt 204 can be prevented from reacting with oxygen and water and from being contaminated.
In the system shown in FIG. 2, the substrates 201 can be set in a larger number as occasion calls. However, experiments made by the present inventors have revealed that it is difficult for the construction of this system to achieve a growth rate which is in-plane uniformly high. FIG. 3 shows in-plane distribution of growth rate where five silicon wafers of 5 inches in diameter are held at intervals of 1 cm and the crystal growth is carried out by means of the above system using an indium solution as the melt and silicon as the crystal to be grown. In FIG. 3, white circles indicate distribution on substrates near to the bottom of the melt; and black circles, distribution on substrates near to the surface layer portion of the melt. Differences between substrates are not so much seen, but only a growth rate of about ⅓ of that at peripheral portion has been attained at the central portion of each substrate.
The growth rate becomes less in-plane non-uniform with a decrease in the cooling rate of the melt, but the growth rate decreases as a whole. Also, the growth rate becomes less in-plane non-uniform with a decrease in the distance between substrates, but substrates that can be set per batch decreases in number, resulting in a decrease in throughput in any case.
The reason why the growth rate is in-plane non-uniform is that any fresh melt can not sufficiently be replenished after the semiconductor materials standing dissolved between the substrates has deposited, and it is considered that, the higher the deposition rate is and the smaller the distance between substrates is, the growth rate is more non-uniform.
In the system shown in FIG. 2, the substrates may be turned during the growth, where the melt containing the silicon in a high concentration is replenished between the substrates, so that the growth rate can be made uniform with ease. This, however, makes it necessary for an up-and-down rod 209 of the substrate-supporting means to make both up-and-down movement and rotational movement. In an attempt to keep the inside of the growth heater hermetic with such construction, the mechanism of the substrate-supporting means must be made large-sized and complicated.
Accordingly, in order to move the melt and the substrate relatively, the substrate may be set stationary and the crucible may be rotated. Rotating a high-temperature crucible is commonly done in single-crystal draw-up systems of the Czochralski method. Techniques which employ this method have already been established.
More specifically, Japanese Patent Application Laid-Open No. 7-315983 discloses a proposal of a case in which the rotation of a crucible is applied in a liquid phase growth system. Setting the substrate stationary and rotating only the crucible can make the substrate-supporting means greatly simple, and is advantageous especially for large-sized liquid phase growth systems. However, in the method in which the crucible is rotated, even though the in-plane distribution of growth rate is relatively good, the growth rate tends to be non-uniform between substrates when the substrates are set in a large number, and any sufficient throughput has not been achieved.